1. Field of the Invention
The present invention relates to a Liquid Crystal Display, and more particularly, to a Liquid Crystal Display with in-cell touch panel.
2. Description of the Prior Art
One type of touch panel, “In-cell touch panel,” comprises many photo elements capable of sensing the light and thus inducing photo-induced currents. By sensing the variance of the photo-induced current, the coordinates of the user's touched points can be determined.
FIG. 1 shows a conventional photo element 100 of an In-cell touch panel. The photo element 100 comprises a photo thin-film transistor 110 and a switch thin-film transistor 130. The source 136 of the switch thin-film transistor 130 is connected to a readout line 140; the gate 132 of the switch thin-film transistor 130 is connected to a switch line 150; the drain 134 of the switch thin-film transistor 130 is connected to the source of the photo thin-film transistor 110. Further, The gate 112 and drain 114 of the photo thin-film transistor 110 are connected to a bias line 120, which provide voltages to the photo thin-film transistor 110. When the switch thin-film transistor 130 is opened after receiving a voltage given by the first switch line 150, a photo-induced current generated by the photo thin-film transistor 110 will be transmitted through the switch thin-film transistor 130 and read out via the readout line 140. Generally, the value of the photo-induced current is proportional to the illumination of the light illuminated on the photo thin-film transistor 110. When the user touches the touch panel by a finger or using a light pen to illuminate the touch panel, the illumination of the light of the touch site will be decreased or increased. Thus the value of the photo-induced current is changed, and therefore the point of the touch event can be determined.
The In-cell touch panel is arranged in the thin-film transistor substrate, which includes a pixel array. Some pixels of the pixel array are regularly embedded with an above-mentioned photo element 100 and thus readout pixels are formed.
FIG. 2 shows a conventional readout pixel 200. The readout pixel 200 comprises a pixel element 210 and an above-mentioned photo element 100. A bias line 120 is employed for not only providing a reference voltage to the storage capacitor Cst of the pixel element 210 but also providing a voltage to drive the photo-induced current to be read out. In addition, the switch line 150 of FIG. 1 corresponds to scan lines Gn-1, Gn of FIG. 2 and Dm-1, Dm denote data lines for providing voltages written to the pixel element 120.
Under operation, the switch thin-film transistor 130 of the photo element 100 is typically shadowed but the photo thin-film transistor 110 of the photo element 100 is illuminated; therefore, after a long-term illumination, the reliability of the photo element 100 will be decreased, the sensitivity will be reduced, and the value of the photo-induced current will be decayed. The worse decay of the photo-induced current reduces the signal to noise (S/N) ratio and may result in incorrect point of touch event to be determined. Hence, the reliability of the photo element 100 after a long-term illumination is an important issue.
When the photo element is operated at office, the illumination of the environment is typically about 300 to 500 lux; when the photo element is operated at the outdoor, the illumination of the environment is typically about 1000 lux (a cloudy day) to 30000 lux (a sunny day). FIG. 3 and FIG. 4 show an I-V curve of a photo element after a long-term illumination, wherein FIG. 3 shows that the photo element is operated at indoor, dark environment, FIG. 4 shows that the photo element is operated at 1500 lux, Vd denotes voltages (unit: volt, V) given to the gate and drain of the photo thin-film transistor, and Is denotes currents (unit: ampere, A) measured at the source of the photo thin-film transistor. The bias voltage applied to the drain of the photo thin-film transistor is 2.63 V in FIG. 3 and 1.95 V in FIG. 4, and both of the FIG. 3 and FIG. 4 have the same duty cycle 1/600.
As shown in FIG. 3, when a voltage, for example, 6V, is stressed to the gate of the photo thin-film transistor for 2 hours and more, the photo-induced current is initially at 77.2 μA then decayed to 70.0 μA and maintained at 70.0 μA; therefore the reliability is acceptable when the photo element is operated at dark environment. By contrast, as shown in FIG. 4, when a voltage, for example, 6V, is stressed to the gate of the photo thin-film transistor for 231 hours, the photo-induced current is decayed about 45.28%; therefore the reliability is unacceptable when the photo element is operated at 1500 lux. The worse decay of the photo-induced current will cause the incorrect point of touch event to be determined.
Therefore, it would be advantageous to liquid crystal display device having novel photo elements that can overcome the defects of the prior art.